Advances in terrain data collection have spurred the need for new tools to work with this data to provide reliable, precise hydrologic and hydraulic models. More accurate sources are making new types of data with larger file sizes available, but previous tools are not optimized for working with the larger volume of this new data. The new data is forcing the development of better systems to handle storing and accessing these large terrain files, as well as new methods for converting the raw data into a useful format that an engineer can use in model generation. Other tools that are needed to deal with the new types of terrain data include methods for generating three-dimensional breaklines along a stream channel based on adjacent elevations, for testing the accuracy of new data, and for creating accurate streamlines through watersheds.
One major problem with terrain models based on elevation data is determining how to make such models hydrologically correct. Natural depressions are present in any raw terrain data format, and determining how to route water or liquid flow out of these areas is difficult. Software, such as the TOPAZ system of the U.S. Department of Agriculture, is known that has attempted to solve this problem by extracting relevant data from raster Digital Elevation Models (DEM) and modifying it. Modified DEMs can be used, for example, for any type of terrain modeling and visualization. Software such as TOPAZ modifies DEM data that is suitable for use by an independent Geographic Information System (GIS). The data is produced by the known D8 method pursuant to which watersheds are divided into cells. Flow vectors are calculated indicating the flow direction from a source cell to one of eight adjacent cells based on the adjacent cell having the steepest downstream slope in relation to the source cell. Once a flow vector has been determined for every cell in a watershed, the model for that watershed is considered to be hydrologically correct. Flow vectors can be used for a variety of purposes including, but not limited to streamline generation, raindrop flowpath tracing, drainage area calculation and determination of the extents of hazardous waste spills.
Unfortunately, the resulting model may be imprecise due to the manner of its handling of depressions and their inherent characteristics. Depressions are areas where the source cell is lower than all eight of its neighbors. These are depressions in the DEM where water will pond. Some programs raise the elevation of the depression digitally in an effort to determine flow direction. As a result, such programs artificially fill all depressions so that inaccurate elevations are created below the calculated outlets.
Another major problem in watershed DEM's is the way in which peaks are handled. Peaks are cells in which there are multiple best directions for the water to flow based on D8 criteria. Some programs solve this problem by applying a purely random solution, but that does not guarantee duplicate results for subsequent runs.
Still a further problem is that programs like TOPAZ have the option of breaching depressions, but they limit the extent of the hydrologic corrections to two DEM cells. This might be acceptable for coarse DEM's with a large cell size, such as 50 feet, but finer DEM's often require more detailed methods for fixing depressions.
Finally, other methods of handling hydrological corrections do not appear capable of handling tiled data. Programs such as TOPAZ are designed to work in a manner that involves processing the entire study area at once, however there is a limit to the maximum number of DEM cells that can be examined at one time. Dividing the study area into different tiled regions and solving each one separately can circumvent this, but the problem arises when corrections are required to span across multiple tiles. If a flat area lies along the border of two tiles and is solved in one manner on the first tile, but in a different manner on the second tile, there is no guarantee that the resulting flow vectors of the two tiles will agree. This often results in cases where flow vectors point at each other along tile boundaries, creating an unsolvable situation that we call an infinite loop.
What is needed, therefore, is a method of efficiently handling the extremely large data sets being provided by available analysis techniques while also producing a watershed model which accurately portrays flow directions through depressions, flat areas and peaks without degrading the raw data from which such flow directions are generated. Similarly, such a method should be capable of providing flowpaths that extend far beyond the edge of depressions, flat areas and peaks across a major portion of the watershed and should enable the handling of tiled data sets.